1. Field of the Invention
The present invention relates to a lock-up mechanism for torque converter, which is used in an automatic transmission for a vehicle or the like, and in particular to an improvement in a friction slide surface of the lock-up mechanism.
2. Description of the Related Art
In general, a torque converter used in a vehicle automatic transmission is composed of a pump (impeller) directly coupled to a crankshaft of an engine, a turbine directly coupled to an output shaft, and a stator located therebetween.
The pump rotates upon rotation of the engine, and as a result, oil (ATF) flows from the pump into the turbine, impinging upon turbine blades so as to effect impact force which rotates the turbine, and accordingly, the output shaft is rotated. At this stage, the stator circulates the fluid between the pump and the turbine in order to increase the energy applied to the turbine.
Thus, the torque converter offers such an advantages that a vehicle enable smooth start, acceleration and deceleration, but raises such a disadvantage that its transmission efficiency is low since a power is transmitted through the intermediary of the fluid. In view of this point, when the vehicle speed exceeds a predetermined value, the stream of the fluid in the torque converter is changed by a hydraulic control mechanism which is not shown so as to press a clutch facing (friction surface) against a front cover of the torque converter. At this time, a lock-up clutch is changed over between a turn-on state and a turn-off state under hydraulic control.
As a result, the engine is mechanically coupled with drive wheels, direct thereto, and accordingly, energy loss can be decreased, thereby it is possible to improve fuel consumption. Thus, almost every of presently used automatic transmissions has a lock-up mechanism (lock-up clutch) incorporated in a torque converter.
Further, in order to materialize low fuel consumption with a high degree of efficiency throughout an entire vehicle speed range including a low vehicle speed range and a high vehicle speed range, these years, there has been dominantly used the so-called slip lock-up control, that is, such lock-up control that a slip rate is maintained at a predetermined rotational speed. The slip lock-up control can suppress vibration and noise even in a low vehicle speed range, which have been difficult to be suppressed by a direct-coupled clutch heretofore used, and accordingly, it is possible to improve the fuel consumption.
Japanese Patent No. 2680632 discloses a lock-up clutch which is formed with oil grooves on the side in opposite to a clutch facing or a friction surface. That is, grooves are formed between a friction liner and the outer surface of the lock-up clutch to which the friction liner is fixed, in order to sufficiently feed lubrication oil that permeates into the entire friction surface.
Japanese Patent Laid-Open No. 10-318306 discloses a such a configuration that a clutch facing or a friction surface is formed therein with a circular outer circumferential groove, a circular inner circumferential groove and a corrugated groove between the outer and inner peripheral grooves in order to enhance its durability and judder resistance. These three grooves are communicated with one another so as to define passages through which-fluid can flow from the inner diameter side to the outer diameter side.
The friction slide surfaces of the lock-up clutch and the front cover fall almost in a slip condition in which they slips, relative to each other. However, the friction surface of a conventional lock-up clutch and the inner surface of the front cover have a relatively low degree of flatness, and accordingly, they exhibit an unstable frictional slide condition. Thus, there has been raised such a problem that judder vibration is likely to occur. In particular, as to slip lock-up control which is recently prosperously used, it has been earnestly desirable to solve the above-mentioned problem.
In the configuration disclosed in the Japanese Patent No. 2680632, the lock-up facing is formed therein with grooves for sufficiently feeing lubrication oil which permeate the entire friction surface, between the friction liner and the outer surface of the lock-up clutch to which the friction surface is fixed, the grooves being not formed in the outer surface of the friction liner but inside of the friction lines. Thus, the above-mentioned problem can hardly be solved.
The Japanese Patent Laid-Open No. 10-318306 discloses the circular outer circumferential groove, the circular inner circumferential groove and the corrugation groove between the inner and outer circumferential grooves, which are formed in the facing or the friction surface. However, since these three grooves are communicated with one another, the fluid flows from the inner diameter side to the outer diameter side, and accordingly, hydraulic pressure is relieved when the clutch is engaged. Thus, it is difficult to maintain a stable slip condition.